[Trent M. Wyatt]’s CPUVolt library provides a fast way to measure voltage using no external components, and no I/O pin. It only applies to certain microcontrollers, but he provides example Ar… ...

If you would like to build your very own benchtop power supply you might be interested in a DIY tutorial for Supplino. A versatile compact Arduino power supply capable of providing from 4 to 40 ...

On page 4, there is an "IDE Power" connector. It looks a lot like a 4 pin CPU fan connector, but it (obviously) provides different voltages that are appropriate for 3.5" IDE drives (5V, 12V) and ...